<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>快递单号查询案例</title>
    <style>
        body{
            padding: 0;
            margin: 0;
        }
        .search {
            position: relative;
            width: 178px;
            margin: 100px;
        }
        .con {
            display: none;
            position: absolute;
            top: -40px;
            width: 171px;
            border: 1px solid rgba(0,0,0,.2);
            box-shadow: 0 2px 4px rgba(0,0,0, .2);
            padding: 5px 0;
            font-size: 18px;
            line-height: 20px;
            color: #333;
        } 
        .con::before{
            content: '';
            width: 0;
            height: 0;
            position: absolute;
            top: 28px;
            left: 18px;
            border: 8px solid #000;
            border-style: solid dashed dashed;
            border-color: #fff transparent transparent;
        }
    </style>
</head>
<body>
    <div class="search">
        <div class="con">123</div>
        <input type="text" placeholder="请输入快递单号" class="jd">
    </div>
    <script>
        var con = document.querySelector('.con')
        var jdinput = document.querySelector('.jd')
        // 不能用keydown 和keypress 不然事触发的时候还没有落入文本框  keyup 触发的时候文字已经落入文本框
        jdinput.addEventListener('keyup',function() {
            if (this.value == '') {
                con.style.display = 'none'
            }
            else{
                con.style.display = 'block'
                con.innerHTML = this.value;
            }
        })
        // 失去焦点 隐藏盒子
        jdinput.addEventListener('blur',function() {
            con.style.display = 'none';
        })
        // 获得焦点 显示盒子
        jdinput.addEventListener('focus',function() {
            if(this.value !== '') {
                con.style.display = 'block';
            }
        })
    </script>
</body>
</html>